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Jerry Coffin
Jerry Coffin
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I wonder if there isn't a minor mistake in the CPU being discussed in one of those.

It's very easy to see the 68K as nearly a direct descendant of the PDP-11. The 68K has separate data and address registers, but is programming it is mostly quite similar to programming a PDP-11.

I'd say the 6800 is (much) closer to a PDP-8. If memory serves, the 6800 has two accumulators and an index register (and PC and stack pointer). My recollection of the PDP-8 isn't quite as clear, but I think it had one accumulator, an index register, a "memory transfer register" (and a PC, but no stack pointer). So, both use an accumulator for most instructions, use a dedicated register for indirect transfers to/from memory, and so on.

On the other hand, it seems to me like it takes some thought to see the similarity between the PDP-8 and the 6800, where the similarity between the 68K and the PDP-11 always struck me as almost exceedingly obvious.

I wonder if there isn't a minor mistake in the CPU being discussed in one of those.

It's very easy to see the 68K as nearly a direct descendant of the PDP-11. The 68K has separate data and address registers, but is programming it is mostly quite similar to programming a PDP-11.

I'd say the 6800 is (much) closer to a PDP-8. If memory serves, the 6800 has two accumulators and an index register (and PC and stack pointer). My recollection of the PDP-8 isn't quite as clear, but I think it had one accumulator, an index register, a "memory transfer register" (and a PC, but no stack pointer). So, both use an accumulator for most instructions, use a dedicated register for indirect transfers to/from memory, and so on.

On the other hand, it seems to me like it takes some thought to see the similarity between the PDP-8 and the 6800, where the similarity between the 68K and the PDP-11 always struck me as almost exceedingly obvious.

I wonder if there isn't a minor mistake in the CPU being discussed in one of those.

It's very easy to see the 68K as nearly a direct descendant of the PDP-11. The 68K has separate data and address registers, but programming it is mostly quite similar to programming a PDP-11.

I'd say the 6800 is (much) closer to a PDP-8. If memory serves, the 6800 has two accumulators and an index register (and PC and stack pointer). My recollection of the PDP-8 isn't quite as clear, but I think it had one accumulator, an index register, a "memory transfer register" (and a PC, but no stack pointer). So, both use an accumulator for most instructions, use a dedicated register for indirect transfers to/from memory, and so on.

On the other hand, it seems to me like it takes some thought to see the similarity between the PDP-8 and the 6800, where the similarity between the 68K and the PDP-11 always struck me as almost exceedingly obvious.

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Jerry Coffin
Jerry Coffin
  • 5.9k
  • 19
  • 29

I wonder if there isn't a minor mistake in the CPU being discussed in one of those.

It's very easy to see the 68K as nearly a direct descendant of the PDP-11. The 68K has separate data and address registers, but is programming it is mostly quite similar to programming a PDP-11.

I'd say the 6800 is (much) closer to a PDP-8. If memory serves, the 6800 has two accumulators and an index register (and PC and stack pointer). My recollection of the PDP-8 isn't quite as clear, but I think it had one accumulator, an index register, a "memory transfer register" (and a PC, but no stack pointer). So, both use an accumulator for most instructions, use a dedicated register for indirect transfers to/from memory, and so on.

On the other hand, it seems to me like it takes some thought to see the similarity between the PDP-8 and the 6800, where the similarity between the 68K and the PDP-11 always struck me as almost exceedingly obvious.